1. Field of the Invention
The present invention relates to nanometer-sized coated metal fine particles which have excellent dispersibility in a solvent and develop favorable conductive properties when sintered at a low temperature on a flexible print substrate or the like, particularly a manufacturing method of coated silver fine particles, and coated metal fine particles manufactured using the above method.
2. Description of Related Art
The remarkable advancement of electronic devices in recent years has been supported by the progress of electronic components, such as semiconductor devices, and the significant development of printed-wiring boards on which the electronic components are mounted. In addition, since the majority of electronic devices are reduced in size, thickness and weight, and, furthermore, there is a demand for improvement in productivity, a variety of corresponding efforts and improvements are further required for printed-wiring boards as well. Particularly, in order to do so, there is a demand for an increase in speed and density of mounting of a material for forming conductive wires of electronic components.
In such a situation, attempts are being made to form fine metal wires not only on polyimide, which has already been in use as a flexible printed-wiring substrate, but also on substrates of a variety of easily-processable organic polymers, such as polyethylene terephthalate (PET) and polypropylene, using a material that enables the formation of metal wires at a lower temperature. Particularly, in order to form metal wires using the existing ink jet printing techniques, studies to form wires by manufacturing an ink that is formed by dispersing nano-sized metal fine particles, printing a desired circuit shape using the ink, and then bonding the metal fine particles through sintering or the like, thereby forming a metal thin film are underway (for example, Mitsuru Kawazome et al., The MICROMERITICS No. 50, 27 to 31 (2006/2007)). In addition, in the final stage of a device, there are frequent cases in which a thermal treatment is strongly ruled out in repairing techniques that cover defects in fine wires, and, in such cases as well, there is a demand for a nano-sized silver fine particle dispersion fluid solution that sinters at an extremely low temperature of 100° C. or lower.
Of the past studies regarding nano-sized metal fine particles, for example, Japanese Laid-Open Patent Publication No. 2005-298921 discloses a manufacturing method of composite metal ultrafine particles in which two or more transition metal salts and amine compounds are thermally treated in an inert atmosphere. The document discloses that ultrafine particles having a nanometer (nm)-level particle diameter are formed, dispersed in a rather non-polar solvent, such as toluene, hexane, ligroin, petroleum ether or ethyl ether, and sintered using a thermal treatment at 200° C. to 300° C. However, it is unclear whether the particles are conductive. In addition, since the thermal treatment temperature is 200° C. or higher, only highly heat-resistant polyimide or the like can be used as an organic substrate having favorable workability.
In addition, Japanese Laid-Open Patent Publication No. 2008-214591 discloses a metal ink for which a polar solvent containing a silver colloid dispersed in a dispersion medium made up of water and a variety of alcohols is used, and describes that, when this ink is used, a silver conductive film can be formed by heating the ink to near 100° C.
In addition, Japanese Laid-Open Patent Publication No. 2008-214695 discloses that oleylamine, which is an alkylamine, is used as an essential component, oleylamine and the like are combined on silver oxalate and the like so as to form a complex compound, then, the generated complex compound is heated and thermally decomposed, thereby obtaining coated silver fine particles having uniformly-sized diameters. In addition, the document describes that, when a mixture of oleylamine and a predetermined amount of a saturated fatty acid amine is used, the generation of the complex compound becomes easy, and silver fine particles can be manufactured in favorable yield within a short period of time.
Furthermore, regarding the technique described in Japanese Laid-Open Patent Publication No. 2008-214695, Japanese Laid-Open Patent Publication No. 2010-265543 describes that, when the kind of the alkylamine is readjusted, particles can be sintered at near room temperature, which is an extremely low temperature as the sintering temperature of silver, particles can be dispersed at a high concentration in an organic solvent, and coated silver fine particles that can be extremely useful in a variety of uses are manufactured.
According to the conductive ink described in Japanese Laid-Open Patent Publication No. 2008-214591, a silver conductive film can be formed at a relatively low temperature, but a dispersion medium including water is used as a polar solvent, and therefore there is a problem with wettability when the ink is applied particularly to organic electronics, such as organic TFTs, such that the ink is easily repelled on a substrate.
Silver fine particles manufactured using the method described in Japanese Laid-Open Patent Publication No. 2008-214695 have an oleylamine layer on the surface, and therefore it becomes possible to manufacture mono-dispersed silver fine particles; however, on the other hand, there were problems in that the manufacturing process took a long time, and the kinds of available amines were limited. That is, there were problems that the reaction rate was not sufficient when oleylamine and silver oxalate generated a complex compound thereof, and, even in a case in which a mixture of a predetermined amount of a saturated fatty acid amine and oleylamine was used, coated silver fine particles could not be smoothly manufactured at all times. In addition, from the viewpoint of the generation of a complex compound as well, since oleylamine having a relatively high molecular weight was used as an essential component, the coating of silver fine particles to be manufactured became rigid such that particles could not be easily sintered at a low temperature at all times.
In addition, according to the method described in Japanese Laid-Open Patent Publication No. 2010-265543, the above problems were improved, but additional improvement in the manufacturing steps of coated silver fine particles or improvement in the performance of coated silver fine particles manufactured were expected.